. >>> updating you on breaking news out of texas where the former u.s. army psychiatrist has been convicted in the fort hood shoot -- shooting rampage. the trial now goes to the sentencing phase where the death penalty is a possibility. >>> he's been the man in charge of one of our country's most secretive and important agencies for more than a decade. he knows where all the bodies are buried. robert mueller opens up to abc news, including what he sees as the greatest threat we're facing. >> reporter: outgoing fbi director robert mueller is offering a sober assessment. he told pierre thomas what keeps him up at night. >> the possibility of a plane being taken down. a weapon of mass destruction in the hands of a terrorist. >> reporter: mueller warned about another danger, americans joining islam mick radicals. do you believe some americans have made their way to syria? yes. >> reporter: he says fror groups and individuals have -- terror groups and individuals have my great to many areas in the middle east. >> it changed from september 11th where it was al